A conformational behaviour of pepsin depending on pH and temperature was studied by circular dichroism, differential UV-spectroscopy, calorimetry and enzymatic hydrolysis kinetics. A subtile conformational transition of the enzyme accompanied by changes in the physico-chemical and enzymatic properties of the protein was observed within the temperature interval of 15--40 degrees and within the pH range of 1,1--5,6. The range of pepsin heat denaturation was studied. A diagram of pepsin conformational states under different values of pH and temperature was built.